Business and consumers use a wide array of wireless devices, including cell phones, wireless local area network (LAN) cards, global positioning system (GPS) devices, electronic organizers equipped with wireless modems, and the like. The increased demand for wireless communication devices has created a corresponding demand for technical improvements to such devices. Generally speaking, wireless system designers attempt to minimize the cost of conventional radio receivers while improving the performance of such devices. Performance improvements include, among other things, lower power consumption, greater range, increased receiver sensitivity, lower bit error rates (BER), higher transmission rates, and the like.
Signal fading due to variations in channel characteristics is a major factor limiting the performance of modern mobile wireless communication systems. To compensate for signal fading, many modern code division multiple access (CDMA) networks use diversity techniques to transmit multiples copies of a signal over a channel to a mobile station. In the mobile station, a RAKE receiver uses multiple baseband correlators to individually process several signal multipath components. The correlator outputs are then combined to achieve improved performance.
However, a RAKE receiver assumes that the channel variations over time are relatively slow. This may not be the case if the mobile station is moving relatively quickly. Channel variations due to Doppler effects caused by the relative motion of the base station transmitter and the mobile station receiver often become significant. To correct this, receivers have been developed that use minimum mean squared error (MMSE) channel estimation filters that require knowledge of the specific Doppler frequency and the specific signal-to-interference ratio (SIR) level. Unfortunately, if the Doppler frequency or the SIR level changes, the filter also changes. Thus, the RAKE receiver requires a Doppler estimator and each finger of the RAKE receiver requires its own SIR estimator.
Therefore, there is a need in the art for improved radio frequency (RF) receivers. In particular, there is a need for improved channel estimation filters for use in RAKE receivers. More particularly, there is a need for a MMSE channel estimation filter that is not specific to a particular Doppler frequency or SIR level provided the SIR is maintained within a reasonable limit by use of downlink power control.